I have a motorised 3-way valve in my central heating system. Today I noticed the valve is clicking every second or so while the lever is in auto. If I move it to manual, it stops.

It is working though as I have hot water and heating.

Is the valve failing? It's about 11 years old.

It sounds like its on its way out pal because you say its ok when you manually do it and only in auto its clicking,
It could be a few things really.... a worn spigot,micro switch or the spring could be damaged and is having issues sending the motorized valve back in to position,
You never said what make you have but Found some tips for a honeywell valve but if you do a search for '3 way motorized boiler valve clicking' then they is lots of people thats had this issue and may help you understand the fault a little more,
Hope this helps.
http://www.lovekin.net/honeywell-mot...ve-faults.html

Thanks, BrightSparks! Pretty sure it's a Honeywell one but I'll check.

I've left the valve in manual for now. I guess this means if I put the hot water on I'll get heating too?